Process control systems typically include field devices and control system components. The field devices may include sensors for measuring the value of a process variable or the status of a process component. The field devices may also include actuators for controlling process equipment such as valves, and relays for controlling process equipment such as motors.
The control system components may include process controllers, safety and alarm monitoring applications, asset management tools, configuration tools, and process historian applications. The field devices and control system components typically communicate with each other via wired connections using a communication protocol.
In some communication protocols, such as the Highway Addressable Remote Transducer (HART) protocol, a separate pair of wires connects each field device to a control system. Such a wire pair may carry an analog control current representing a process variable or a desired actuator position. A low voltage frequency shift keying (FSK) digital signal may be superimposed on the analog control current to enable communication of other information between the HART field device and the control system. Such other information may include configuration information, other process variable information, and device status information.
A HART multiplexer may be used to simplify the wiring of such a system. A multiplexer unit may communicate with individual HART field devices using the combined analog and FSK digital communication protocol described above. A serial digital link may then be used to communicate between the multiplexer unit and a control system using a related digital communication protocol.
In other communication protocols, a control system may communicate with one or more field devices over a wire pair carrying serial digital signals. Foundation Fieldbus (FF) H1 and Profibus are two examples of such a protocol. The number of field devices that may be supported by a single wire pair is limited. Both H1 and Profibus have related protocols (FF HSE and Profinet, respectively), which allow the control system to communicate with an increased number of field devices over a single connection.
Such control systems, however, continue to require that system owners install wired connections of one sort or another between control system components and field devices. This requirement contributes significantly to the cost and complexity of initial installation and expansion of process control systems.